1. Field of the Invention
The present invention relates to a method for manufacturing a martensite stainless steel pipe having excellent corrosion resistance. The invention may be used in manufacturing petroleum and natural gas pipelines.
2. Description of the Related Art
Almost all of the petroleum and natural gas in the world which can be easily extracted has been recovered. Therefore, more and more development is taking place in severe environments, particularly in wells deep underground, frigid locations and offshore sites.
Significant quantities of carbonic acid gas are often contained in petroleum and natural gas recovered from wells located in these severe environments, thereby causing great corrosion of carbon steel or low alloy steels. To cope with this problem, an inhibitor is conventionally added to such steels as a corrosion prevention means.
However, inhibitors not only increase the cost of the steels, they are not effective at high temperatures. Steels which are corrosion resistant without inhibitors, such as martensite stainless steel containing 13% Cr, are now widely used in place of steels containing inhibitors.
API Standards require that a line pipe be composed of 12% Cr martensite stainless steel containing a reduced amount of C. However, this steel is almost never employed as line pipe because preheating and postheating are required for peripheral welding, which tremendously increases costs. Further, toughness in the welded portions is poor. Consequently, two-phase stainless steel having an increased amount of C as well as Ni and Mo is often used as corrosion resistant line pipe because it possesses excellent weldability and corrosion resistance. However, the two-phase stainless steel is expensive and often exceeds the requirements dictated by conditions in some wells.
A method of manufacturing a martensite stainless steel line pipe is disclosed in, for example, Japanese Patent Application Laid-Open No. 4-99128 as a means for overcoming the above problem. Disclosed therein is a method of manufacturing a line pipe of 13% Cr stainless steel which comprises 1.2-4.5% Cu and reduced contents of C and N. After the 13% Cr stainless steel is formed into a pipe, the pipe is cooled at a quenching cooling speed higher than that effected by water. As a result, the stainless steel pipe exhibits excellent corrosion resistance even in a corrosive environment containing a carbonic acid gas, has low hardness in a welding-heat-affected zone and avoids quench cracking. However, this method still fails to produce sufficient toughness in the welding-heat-affected zone (HAZ zone).
An object of the present invention is to provide a method of manufacturing martensite stainless steel pipe having high surface corrosion resistance, high pitting resistance, excellent weld cracking resistance and welded portion toughness.